Pepsin is the enzyme that works best in the acidic environment of the stomach, according to Eyclopædia Britannica. The main purpose of the enzyme is to break down the food proteins into peptides.
Eyclopædia Britannica explains that pepsin is stored in the lining of the stomach as pepsinogen. Once released into the stomach and mixed with hydrochloric acid, inactive pepsinogen breaks down to active pepsin.
J. Stein Carter at UC Clermont College notes that Pepsin needs an acidity level between pH 1.5 and pH 2.5 to work effectively. If the stomach acidity is altered through the use of antacids, the enzyme’s ability to digest protein is drastically hindered.